The present invention relates generally to inkjet printing devices, and more particularly to an inkjet printhead for thermal inkjet printing devices that provides a high density of ink drop generators for ejecting ink from the printhead.
The art of inkjet printing technology is relatively well developed. Conmmercial products such as computer printers, graphics plotters, copiers, and facsimile machines successfully employ inkjet technology for producing hard copy printed output. The basics of the technology has been disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No.1 (February 1994) editions. Inkjet devices have also been described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices (R. C. Durbeck and S. Sherr, ed., Academic Press, San Diego, 1988, chapter 13).
A thermal inkjet printer for inkjet printing typically includes one or more translationally reciprocating print cartridges in which small drops of ink are ejected by a drop generator towards a medium upon which it is desired to place alphanumeric characters, graphics, or images. Such cartridges typically include a printhead having an orifice member or plate that has a plurality of small nozzles through which the ink drops are ejected. Beneath the nozzles are ink firing chambers, enclosures in which ink resides prior to ejection by an ink ejector through a nozzle. Ink is supplied to the ink firing chambers through ink channels that are in fluid communication with an ink reservoir, which may be contained in a reservoir portion of the print cartridge or in a separate ink container spaced apart from the printhead.
Ejection of an ink drop through a nozzle employed in a thermal inkjet printer is accomplished by quickly heating the volume of ink residing within the ink firing chamber with a selectively energizing electrical pulse to a heater resistor ink ejector positioned in the ink firing chamber. At the commencement of the heat energy output from the heater resistor, an ink vapor bubble nucleates at sites on the surface of the heater resistor or its protective layers. The rapid expansion of the ink vapor bubble forces the liquid ink through the nozzle. Once the electrical pulse ends and an ink drop is ejected, the ink firing chamber refills with ink from the ink channel and ink reservoir.
The minimum electrical energy required to eject an ink drop of a reliable volume is referred to as "turn-on energy". The turn-on energy is a sufficient amount of energy to overcome thermal and mechanical inefficiencies of the ejection process and to form a vapor bubble having sufficient size to eject an amount of ink (generally determined by the design parameters of the firing chamber) from the printhead nozzle. Conventional thermal inkjet printheads operate at a firing energy slightly greater than the turn-on energy to assure that drops of an uniform size are ejected. Adding substantially more energy than the turn-on energy generally does not increase drop size but does deposit excess heat in the printhead.
Following removal of electrical power from the heater resistor, the vapor bubble collapses in the firing chamber in a small but violent way. Components within the printhead in the vicinity of the vapor bubble collapse are susceptible to fluid mechanical stresses (cavitation) as the vapor bubble collapses, thereby allowing ink to crash into the ink firing chamber components. The heater resistor is particularly susceptible to damage from cavitation. One or more protective layers are typically disposed over the resistor and adjacent structures to protect the resistor from cavitation and from chemical attack by the ink. One protective layer in contact with the ink is a mechanically hard cavitation layer that provides protection from the cavitation wear of the collapsing ink. Another layer, a passivation layer, is typically placed between the cavitation layer and the heater resistor and its associated structures to provide protection from chemical attack. Thermal inkjet ink is chemically reactive, and prolonged exposure of the heater resistor and its electrical interconnections to the ink will result in a degradation and failure of the heater resistor and electrical conductors. The foregoing protection layers, however, tend to increase the inherent turn-on energy of the heater resistor required for ejecting ink drops due to the insulating properties of the layers.
Some of the energy that is deposited by the heater resistors is not removed by the ejected ink drop as momentum or increased drop temperature, but remains as heat in the printhead or the remaining ink. As the temperature increases, the ink drop size can change and at some temperature, the printhead will no longer eject ink. Therefore it is important to control the amount of heat that is generated and that remains in the printhead during a printing operation. As more resistors are activated with higher frequencies of activation and are packed with greater density in the printhead, significantly more heat is retained by the printhead. Consequently, there must be a reduction in the amount of energy input to the printhead for higher frequencies and greater drop generator densities to be realized.
The heater resistors of a conventional inkjet printhead comprise a thin film resistive material disposed on an oxide layer of a semiconductor substrate. Electrical conductors are patterned onto the oxide layer and provide an electrical path to and from each thin film heater resistor. Since the number of electrical conductors can become large when a large number of heater resistors are employed in a high density (high DPI--dots per inch) printhead, various multiplexing techniques have been introduced to reduce the number of conductors needed to connect the heater resistors to circuitry disposed in the printer. See, for example, U.S. Pat. No. 5,541,629 "Printhead with Reduced Interconnections to a Printer" and U.S. Pat. No. 5,134,425, "Ohmic Heating Matrix". Each electrical conductor, despite its good conductivity, imparts an undesirable amount of resistance in the path of the heater resistor. This undesirable parasitic resistance uselessly dissipates a portion of the electrical energy which otherwise would be available to the heater resistor thereby contributing to the heat gain of the printhead. If the heater resistance is low, the magnitude of the current drawn to nucleate the ink vapor bubble will be relatively large resulting in the amount of energy wasted in the parasitic resistance of the electrical conductors being significant relative to that provided to the heater resistor. That is, if the ratio of resistances between that of the heater resistor and the parasitic resistance of the electrical conductors (and other components) is too small, the efficiency (and the temperature) of the printhead suffers with the wasted energy.
The ability of a material to resist the flow of electricity is a property called resistivity. Resistivity is a function of the material used to make the resistor and does not depend upon the geometry of the resistor of the thickness of the resistive film used to form the resistor. Resistivity is related to resistance according to: EQU R=.rho.L/A
where R=resistance (Ohms); .rho.=resistively (Ohm-cm); L=length of resistor; and A=cross sectional area of resistor. For thin film resistors typically used in thermal inkjet printing applications, a property commonly known as sheet resistance (R.sub.sheet) is commonly used in analysis and design of heater resistors. Sheet resistance is the resistivity divided by the thickness of the film resistor, and resistance is related to sheet resistance by: EQU R=R.sub.sheet (L/W)
where L=length of the resistive material and W=width of the resistive material. Thus, resistance of a thin film resistor of a given material and of a fixed film thickness is a simple calculation of length and width for rectangular and square geometries.
Most of the thermal inkjet printers available today use square heater resistors that have a resistance of 35 to 40 .OMEGA.. If it were possible to use resistors with higher values of resistance, the energy needed to nucleate an ink vapor bubble would be transmitted to the thin film heater resistor at a higher voltage and lower current. The energy wasted in the parasitic resistances would be reduced and the power supply that provides the power to the heater resistors could be made smaller and less expensive.
As users of inkjet printers have begun to desire finer detail in the printed output from a printer, the technology has been pushed into a higher resolution of ink drop placement on the medium. One of the common ways of measuring the resolution is the measurement of the maximum number of ink dots deposited in a selected dimension of the printed medium, commonly expressed as dots per-inch (DPI). The production of an increased DPI requires smaller drops. Smaller ink drops means a lowered drop weight and lowered drop volume for each drop. Production of low drop weight ink drops requires smaller structures in the printhead. Smaller drops and resultant dots means that more dots must be placed on the medium at a higher rate in order to maintain a reasonable speed of printing, i.e., the number of pages printed per minute. The increased speed of printing requires a higher rate of drop generator heater resistor activation. So, designers of inkjet printheads are faced with the problem of more drop generators (with their associated heater resistors) disposed over a smaller area of printhead being operated at an increased frequency. These requirements produce a higher density of heat and higher temperatures.
One approach to resolving the heat problem has been to increase the size of the semiconductor substrate as a heat spreader and heat sink. This approach, however, leads to an unacceptably higher cost, since processed semiconductor material costs rise exponentially with increased area. Moreover, there is a strong motivation to maintain a constant sized silicon substrate to enable manufacturing of varying printhead performance levels on the same manufacturing equipment. It is possible to control printhead temperature by slowing the rate of heater resistor activation--the duty cycle of the heating pulses can be lower--but this leads to a lower page per minute printing delivery and is unacceptable to the user of the printing device. Thus, what is needed is a solution that enables compact printheads with high density drop generators and high printing throughput but without excessive heat generation within the printhead.